Conventionally, as an exhaust gas purifier of this type, there is disclosed an exhaust purifier that is configured so that an oxidation catalyst for oxidizing a part of NO in an exhaust gas to NO2 is provided in an exhaust pipe of an internal combustion engine, a NOx trapping catalyst for trapping NOx in the exhaust gas in copresence of hydrogen is provided in the exhaust pipe on a downstream side of the oxidation catalyst, a hydrogen supply means for reforming a fuel to generate a reductive gas containing at least hydrogen and supplying this reductive gas between the oxidation catalyst and the NOx trapping catalyst in the exhaust pipe is provided separately from the exhaust pipe; and furthermore, a control means supplies the reductive gas by the hydrogen supply means under conditions including a condition that a temperature of the exhaust gas is in a predetermined range (see, for example, Patent Literature 1). In this exhaust purifier, a first catalyst converter and a second catalyst converter are provided in the exhaust pipe in this order from the upstream side, and a supply port of a fuel reformer is provided between the first catalyst converter and the second catalyst converter in the exhaust pipe. The fuel reformer has a reformed gas introducing passageway, one end side of which is connected to the supply port, a reforming catalyst provided in this reformed gas introducing passageway, and a fuel gas supply device for supplying a fuel gas to the reforming catalyst from the other end side of the reformed gas introducing passageway. This fuel reformer reforms the fuel of the engine by the action of the reforming catalyst, to thereby generate the reformed gas of the reductive gas containing hydrogen (H2), carbon monoxide (CO), and hydrocarbon (HC). It is noted that the fuel reformer is provided with a heating heater containing a glow plug, a spark plug, and the like and is configured so that the reforming catalyst can be heated when the fuel reformer is started. According to this, the temperature of the reformed gas generated by the fuel reformer reaches a high temperature in a range of from about 500° C. to 800° C.
Meanwhile, the reforming catalyst contains at least one metal catalyst component selected from the group consisting of rhodium, platinum, palladium, nickel, and cobalt, and at least one oxide selected from the group consisting of ceria, zirconia, alumina, and titania, or a composite oxide including such a material as a basic composition. This reforming catalyst reforms the fuel gas supplied from the fuel gas supply device, to thereby generate the reformed gas containing hydrogen, carbon monoxide, and a hydrocarbon. Also, the reformed gas generated with the reforming catalyst passes through the reformed gas introducing passageway and is supplied from the supply port into the exhaust pipe. Furthermore, a reformed gas introducing valve for controlling a flow rate of the reformed gas, which passes through this reformed gas introducing passageway and is introduced into the exhaust pipe, is provided in an intermediate portion of the reformed gas introducing passageway. This reformed gas introducing valve is connected to an ECU via an actuator, and a supply amount of the reformed gas to the exhaust pipe via the reformed gas introducing passageway is controlled by the ECU.
In the thus configured exhaust purifier, since the oxidation catalyst for oxidizing a part of NO in the exhaust gas to NO2 is provided on the upstream side of the NOx trapping catalyst for trapping NOx in the exhaust gas in copresence of hydrogen, a mixed gas of NO2-rich NO and NO2 can be supplied to the NOx trapping catalyst. As a result, a NOx trapping performance by the NOx trapping catalyst at a low temperature immediately after start of the internal combustion engine can be enhanced. More specifically, even if the temperature of the NOx trapping catalyst is about 50° C. or lower, it is possible to commence trapping of a significant amount of NOx. In this way, NOx in the exhaust gas can be trapped with the NOx trapping catalyst by supplying the reductive gas containing hydrogen to the NOx trapping catalyst whose NOx trapping performance has been enhanced in this way, under conditions including a condition that the temperature of the exhaust gas is in a predetermined range, more specifically in a low temperature region, and therefore, a NOx purification rate at the cold machine time immediately after start of the internal combustion engine can be enhanced. Also, since the hydrogen supply means is provided separately from the exhaust pipe, even at the cold machine time period, the reductive gas can be stably supplied without causing an accidental fire on the internal combustion engine. Also, by supplying the reductive gas under the conditions including the condition that the temperature of the exhaust gas in the predetermined range which is in the low temperature region, the reductive gas is made to reach the NOx trapping catalyst without burning hydrogen, thereby enabling it to contribute to trapping of NOx. Furthermore, the oxidation catalyst is provided on the upstream side of the NOx trapping catalyst to supply the mixed gas of NO and NO2 to the NOx trapping catalyst, and therefore, the NOx trapping performance of the NOx trapping catalyst at the low temperature immediately after start of the internal combustion engine can be enhanced. As a result, it becomes unnecessary to dispose the NOx trapping catalyst in the vicinity of the internal combustion engine.
Meanwhile, there is disclosed an internal combustion engine that is configured so that a turbo supercharger and an exhaust gas purification catalyst are provided in an exhaust passageway, through which an exhaust gas discharged from an exhaust port of a cylinder head passes, in such a manner that the turbo supercharger is located on an upstream side and the exhaust gas purification catalyst is located on a downstream side, and a fuel can be vaporized and supplied to the exhaust gas purification catalyst (see, for example, Patent Literature 2). In this internal combustion engine, a vaporization chamber for vaporizing the fuel by heating with heat of the exhaust gas is provided in a portion of the exhaust passageway on the upstream side of the turbo supercharger, and this vaporization chamber is connected to a portion of the exhaust passageway on the downstream side of the turbo supercharger. Also, the vaporization chamber is integrally provided with a part of an exhaust manifold fixed to the cylinder head in such a manner that an interior of the vaporization chamber and an interior of the exhaust manifold are partitioned by a partition wall, and the above-described partition wall is provided with a communication hole for injecting only a small amount of the exhaust gas into the vaporization chamber.
In the thus configured internal combustion engine, nevertheless the fuel is vaporized utilizing the heat of the exhaust gas, it is possible to avoid the fuel for reducing the catalyst from being introduced into the turbo supercharger. Therefore, nevertheless its structure is simple, it is possible to prevent a turbine or the like of the turbo supercharger from erosion or to prevent properties of the exhaust gas from deterioration on the occasion that the exhaust gas is returned to the exhaust gas to the cylinder. In particular, since the fuel is exposed to the exhaust gas, the fuel can be vaporized rapidly and surely, and since the vaporized fuel is quickly fed to the catalyst by a pressure (positive pressure) of the exhaust gas, there is brought such an advantage that the responsibility is high.
Also, there is disclosed an exhaust gas purifier of a diesel engine in which a vaporization combustion chamber for a reducing agent in which a vaporization chamber for the reducing agent is integrated with a protector so that vaporization and combustion of the reducing agent are not affected by an exhaust gas flow is provided on an upstream side of a catalyst part for denitration of an exhaust gas discharge passage (see, for example, Patent Literature 3). In this exhaust gas purifier, magnetite (Fe3O4) is used as a denitration catalyst, and this magnetite is used in a shape having a large contact area with the exhaust gas, for example, a granulate shape, a pellet shape, a honeycomb shape, etc. Also, as a hydrocarbon which is used as the reducing agent, light oil, heavy oil, benzin, methane, ethane, and the like can be used. Furthermore, in order to increase combustion properties of the reducing agent or for the nitration reaction of the exhaust gas, secondary air is introduced into the exhaust gas discharge passage.
In the thus configured exhaust gas purifier of the diesel engine, when the reducing agent including a hydrocarbon or an alcohol and secondary air (oxygen) are added to the exhaust gas and the mixture is brought into contact with magnetite (Fe3O4), NOx in the exhaust gas is reduced and converted into nitrogen. Also, when the reducing agent such as light oil or heavy oil is burnt with high efficiency in a place where the exhaust gas of the diesel engine always flows, particulate on the catalyst surface is removed, and the denitration of the diesel engine can be efficiently achieved while cleaning the catalyst surface. For that reason, when the vaporization combustion chamber for the reducing agent is provided on the upstream side of the catalyst part disposed in the exhaust gas discharge passageway and when the reducing agent is vaporized in this vaporization combustion chamber, even if the exhaust gas of the diesel engine is discharged, the protector does not hinder the vaporization and combustion of the reducing agent in the vaporization combustion chamber, and therefore, the vaporization and combustion of the reducing agent are performed in the vaporization combustion chamber without hindrance. As a result, by setting up the vaporization combustion chamber capable of surely performing the vaporization of the reducing agent and the combustion of the reducing agent and being free from hindrance of the combustion by the exhaust gas in the exhaust gas passage of the diesel engine, the particulate on the catalyst surface is effectively removed, and the denitration of the exhaust gas of the diesel engine can be efficiently achieved while cleaning the catalyst surface.